PPS fibers have excellent heat resistance, chemical resistance, flame retardance and the like and hence have been expected to permit their use in various application fields such as various kinds of filters, electrical insulating materials and fibers for paper machine canvases.
However, the PPS fibers are still insufficient in strength properties such as tensile strength and knot tenacity, or flex-resistant performance.
Various proposals have heretofore been made in order to improve the mechanical properties, heat resistance, chemical resistance and the like of the PPS fibers.
For example, it is disclosed in Japanese Patent Publication No. 3961/1989 to stretch unstretched PPS filaments at a draw ratio higher than a natural draw ratio as first-stage stretching, and then subject them to either a heat treatment at a temperature of 150.degree.-260.degree. C., which is higher than the stretching temperature in the first-stage stretching, under fixed length or second-stage stretching in the same temperature range to give a total draw ratio of 1-2 times that of the first-stage stretching, thereby improving the mechanical properties, heat resistance and chemical resistance of the filaments.
In Japanese Patent Application Laid Open No. 299513/1987, there is disclosed a process for producing PPS monofilaments improved in tensile strength and knot tenacity by melt-extruding a linear PPS having a melt flow rate of 200 or lower, cooling the extrudate in hot water of at least 60.degree. C., subsequently subjecting the thus-obtained unstretched monofilaments to first stretching at a draw ratio such that a ratio of the first draw ratio to a total draw ratio is lower than 0.88 and then to a multi-stage stretching to give the total draw ratio of 4:1, and then heat-treating them under relaxation in an air bath at 200.degree.-280.degree. C.
It is disclosed in Japanese Patent Application Laid-Open Nos. 229809/1989 and 239109/1989 to melt-spin PPS, stretch the resulting fibers at one stage using a heated member (hot roller), heat set the thus-stretched fibers using another heated member the surface temperature of which is 100.degree.-140.degree. C. and then further heat set the thus-treated fibers using a further heated member the surface temperature of which is in a range of from at least 150.degree. C. to at most the melting point of the PPS, thereby obtaining PPS fibers extremely reduced in fuzzing, filament breaking and end breakage.
However, since the processes according to these known techniques have failed to improve the flex resistance to a sufficient extent, it has been difficult to obtain PPS fibers excellent in tensile strength, knot tenacity and the like and moreover, sufficiently good in flex resistance. Accordingly, there has not been obtained under the actual circumstances any PPS fibers which can satisfactorily meet flexing abrasion resistance and flexing fatigue resistance required urgently for use, for example, as fibers for paper machine canvases.